Melatonin Analogue Shows Potent Anti-Breast Cancer Activity in Lab Tests

A synthetic melatonin analogue has shown a striking multi-pronged attack on estrogen receptor-positive breast cancer cells in laboratory experiments, blocking cell proliferation, migration, and the metabolism of estrogens, although the compound failed to shrink tumors when tested in mice. The findings, published in Discover Endocrinology and Metabolism, come from a research team led by Elif Ince-Erguc and Hande Gurer-Orhan, whose work builds on earlier studies identifying indole-based melatonin analogues as potent aromatase inhibitors.

The compound at the center of the study, 2-methyl-1H-indole-3-carboxaldehyde-(4-chlorophenyl)hydrazone, known simply as 1j, is a hydrazone derivative synthesized from 2-methyl-1H-indole-3-carboxaldehyde and 4-chlorophenyl hydrazine hydrochloride. In previous work by the same group, the compound emerged as the most effective aromatase inhibitor among more than twenty tested 2-methyl indole hydrazones, with a reported half-maximal inhibitory concentration of 8.67 micromolar. The new study set out to expand its mechanistic profile, testing whether 1j could also kill breast cancer cells, stop them from migrating, antagonize the estrogen receptor, and inhibit CYP1B1, an enzyme that converts estrogens into potentially genotoxic metabolites.

In cytotoxicity assays using the MTT method, 1j killed breast cancer cells in a time- and dose-dependent manner. After 24 hours of exposure, the compound showed its highest potency against the murine ER-positive 67NR cell line, with an IC50 of 11 micromolar, while the human ER-positive MCF-7 cell line did not reach 50 percent inhibition even at the highest concentration tested of 100 micromolar. When the exposure period was extended to 72 hours, however, the picture changed dramatically: the IC50 for MCF-7 cells fell to 31 micromolar and for 67NR cells to 5.2 micromolar, while the non-tumorigenic human breast epithelial cell line MCF-10A showed an IC50 of 10.8 micromolar. The researchers note that the differential sensitivity between the human and murine models may reflect inherent biological differences, including variability in estrogen-metabolizing enzymes, although no direct comparative expression analyses were performed, so these interpretations remain exploratory.

Selectivity improved with longer exposure. In colony formation assays, which track the ability of single cells to divide repeatedly over 240 hours, treatment with 50 micromolar 1j eliminated colony formation entirely in both human and murine breast cancer cell lines. At 10 micromolar, colony formation was significantly inhibited in MCF-7 cells, and 67NR cells showed a clear dose-dependent reduction in survival, while MCF-10A cells were unaffected at both 2 and 10 micromolar. Only at the highest concentration did the non-tumorigenic cells also lose their ability to form colonies, indicating that the compound’s selectivity for cancer cells diminishes at higher doses. The team also verified that 1j did not interfere with the MTT assay chemistry itself, ruling out a common source of false-positive results.

Perhaps the most eye-catching result concerns cell movement. At a sub-cytotoxic concentration of 5 micromolar, maintained in low-serum conditions to minimize proliferation-related interference, 1j reduced the migration of MCF-7 cells to 36 percent of vehicle-control levels and that of 67NR cells to just 14 percent in a scratch assay over 24 hours. The murine cells proved more sensitive across the board, consistent with the cytotoxicity data. The authors caution, however, that even though the concentration was chosen as sub-cytotoxic based on short-term viability data, residual cytotoxic or metabolic effects under the low-serum conditions cannot be completely excluded, so the wound-closure reduction should be interpreted with that limitation in mind.

The compound also demonstrated genuine estrogen receptor antagonism. In a two-step E-Screen assay using MCF-7 BUS cells, increasing concentrations of 1j co-incubated with 0.1 nanomolar estradiol reduced estradiol-driven proliferation with an IC50 of 0.9 micromolar. Crucially, when a fixed dose of 1j was combined with increasing estradiol concentrations, cell proliferation was restored in a dose-dependent manner, supporting an ER-mediated mechanism rather than nonspecific toxicity. The authors place this potency in context: it is weaker than the full antagonist fulvestrant, which acts at nanomolar concentrations, and somewhat weaker than tamoxifen, which shows an IC50 of roughly 0.5 micromolar in the same assay, but it sits within the same micromolar range as the clinically used modulator. Compared with the team’s earlier 1-ethyl indole derivatives, which showed an IC50 of 2.89 micromolar, the new compound represents a potency improvement.

Where 1j truly excelled was in blocking CYP1B1. This cytochrome P450 enzyme preferentially converts estradiol and estrone into 4-hydroxylated metabolites that can undergo further oxidation to reactive quinones capable of forming DNA adducts and mutagenic lesions, and elevated CYP1B1 expression has been reported in breast tumor tissues. Using an EROD assay with both phenobarbital-induced rat hepatic microsomes and recombinant human CYP1B1, the researchers found that 1j inhibited microsomal CYP1 activity with an IC50 of 1.9 micromolar and recombinant human CYP1B1 with an IC50 of 0.052 micromolar, the latter roughly 60 times more potent than the comparator drug ketoconazole. Combined with its previously established aromatase inhibition, the compound now targets estrogen signaling, estrogen synthesis, and estrogen metabolism simultaneously, a profile the authors argue positions it as a potential adjuvant candidate alongside established endocrine therapies rather than a replacement for them.

The in vivo results, however, temper the enthusiasm. In a syngeneic ER-positive model using 67NR cells implanted into female Balb/c mice, animals received daily intraperitoneal injections of 1 mg/kg body weight 1j starting four days before tumor inoculation and continuing for 20 days, with tumor growth monitored three times per week until day 32. Under these conditions, treatment did not produce a significant reduction in tumor volume compared with controls, and no notable changes in body weight were observed. The authors emphasize several important caveats: the design was preventive rather than therapeutic, since dosing began before tumor implantation; the 67NR model represents a non-metastatic luminal A-like ER-positive subtype; and the absence of hormone-controlled conditions, such as ovariectomy with defined estradiol supplementation or endocrine comparator arms, means tumor growth in this setting may have been only partly estrogen-dependent, so the in vivo outcome cannot be directly attributed to the ER-mediated mechanisms seen in vitro.

Pharmacokinetic realities may also explain the translational gap. The authors note that clinically reported melatonin plasma concentrations, with maximum values around 0.64 to 0.84 micromolar, are markedly lower than the micromolar concentrations needed to elicit activity in the in vitro assays, suggesting that the exposure levels required may not be readily achievable in vivo. They point to possible ways forward, including optimized or fractionated dosing regimens, alternative routes of administration, and combination strategies with established endocrine therapies. Preclinical evidence indicates that melatonin enhances the inhibitory effects of aromatase inhibitors in ER-positive breast cancer cells, and clinical data suggest that co-administration of melatonin may improve responses to tamoxifen in patients with advanced breast cancer, supporting further evaluation of melatonin analogues in combination settings.

Overall, the study delivers a detailed preclinical characterization of a multi-target indole-based melatonin analogue: selective and time-dependent cytotoxicity toward ER-positive breast cancer cells, strong suppression of long-term colony formation, marked inhibition of cell migration, moderate ER antagonism, and exceptionally potent CYP1B1 inhibition. The authors conclude that these robust in vitro activities did not translate into measurable antitumor efficacy under the tested dosing regimen, and that future studies incorporating pharmacokinetic characterization, optimized in vivo designs, and combination approaches with established endocrine therapies will be required to define the therapeutic potential of compound 1j more fully. For now, the compound remains a laboratory-stage candidate whose promise rests on cell culture data, with the critical test of efficacy in living animals still to be demonstrated.

Subject of Research: Preclinical evaluation of an indole-based melatonin analogue against estrogen receptor-positive breast cancer

Article Title: Beneficial effects of an indole derivative melatonin analogue in estrogen receptor positive breast cancer

Article References: Ince-Erguc, E., Aliyev, A. T., Krager, K., Suzen, S., Aykin-Burns, N., & Gurer-Orhan, H. (2026). Beneficial effects of an indole derivative melatonin analogue in estrogen receptor positive breast cancer. Discover Endocrinology and Metabolism, 2(1), Article 6. https://doi.org/10.1007/s44417-026-00019-2

Image Credits: AI Generated

DOI: 10.1007/s44417-026-00019-2

Keywords: melatonin analogue, breast cancer, estrogen receptor, CYP1B1, aromatase inhibitor, cell migration, E-Screen assay, MCF-7, 67NR, syngeneic tumor model, endocrine therapy, preclinical study